The claudin‐16 channel gene is transcriptionally inhibited by 1,25‐dihydroxyvitamin D

New FindingsWhat is the central question of this study? In the kidney, the bulk of the filtered Mg 2+ is reabsorbed in the thick ascending limb by paracellular conductance, mediated by the tight junction protein, claudin‐16, which is encoded by the gene CLDN16 . The role of 1,25‐dihydroxyvitamin D [1,25(OH) 2 VitD] in renal Mg 2+ handling is unclear. We aimed to explore the molecular mechanisms underlying the effect of 1,25(OH) 2 VitD on claudin‐16‐mediated Mg 2+ transport. What is the main finding and its importance? Paracellular, claudin‐16‐mediated Mg 2+ transport is transcriptionally repressed by 1,25(OH) 2 VitD, probably via a Ca 2+ ‐sensing receptor‐dependent mechanism. This renal effect of 1,25(OH) 2 VitD may serve as an adaptive mechanism to the 1,25(OH) 2 VitD‐induced enteric hyperabsorption of dietary Mg 2+ .Magnesium is reabsorbed in the thick ascending limb by paracellular conductance, mediated by the CLDN16 ‐encoded tight junction protein, claudin‐16. However, the role of 1,25‐dihydroxyvitamin D [1,25(OH) 2 VitD] in renal Mg 2+ handling is unclear. We have shown that Mg 2+ depletion increases and 1,25(OH) 2 VitD inhibits CLDN16 transcription. We have now explored further the molecular mechanisms underlying the effect of 1,25(OH) 2 VitD on claudin‐16‐mediated Mg 2+ transport. Adult mice received parenteral 1,25(OH) 2 VitD or 1,25(OH) 2 VitD combined with either high‐Mg 2+ or low‐Mg 2+ diets. Administration of 1,25(OH) 2 VitD enhanced urinary excretion of Mg 2+ and Ca 2+ . The 1,25(OH) 2 VitD also increased renal Ca 2+ ‐sensing receptor (CaSR) mRNA and decreased renal claudin‐16 and claudin‐19 mRNA and claudin‐16 protein, but did not affect renal claudin‐2 mRNA. The 1,25(OH) 2 VitD reversed the expected increase in claudin‐16 mRNA in Mg 2+ ‐depleted animals. Comparably treated HEK 293 cells showed similar changes in claudin‐16 mRNA, but 1,25(OH) 2 VitD did not alter mRNA of the TRPM6 Mg 2+ channel. A luciferase reporter vector containing 2.5 kb of 5′‐flanking DNA sequence from human CLDN16 (h CLDN16 ) was transfected into HEK 293 and OK cells. The h CLDN16 promoter activity was modestly decreased by 1,25(OH) 2 VitD, but markedly inhibited in HEK 293 cells coexpressing CaSR. Coexpression in OK cells of dominant‐negative CaSR completely abolished inhibition of h CLDN16 promoter activity by 1,25(OH) 2 VitD. The 1,25(OH) 2 VitD‐induced decrease in h CLDN16 promoter activity was attenuated in Mg 2+ ‐depleted HEK 293 cells. In conclusion, 1,25(OH) 2 VitD transcriptionally inhibits claudin‐16 expression by a mechanism sensitive to CaSR and Mg 2+ . This renal effect of 1,25(OH) 2 VitD may serve as an adaptive response to the 1,25(OH) 2 VitD‐induced increase in intestinal Mg 2+ absorption.